1. Field of the Invention
This invention relates to a heat recovery gas turbine plant system supplied with a low-pressure industrial by-product gas, for instance, blast furnace gas, as a fuel, and more particularly to a gas pressure stabilizer for safely returning a high pressure fuel gas on the discharge side of a fuel gas compressor into a low-pressure industrial by-product gas pipeline at the time of emergency shut-down of a gas turbine in such a system. Such gas turbine system can be employed in various process plant such as a paper manufacturing and pulp processing plant, a Portland cement manufacturing plant, a petroleum refinery plant or the like where a by product gas is obtained.
2. Description of the Prior Art
As a power generation system employing a by-product gas fired gas turbine supplied with blast furnace gas as a fuel, there has recently been developed a large scale system in which, as disclosed in Japanese Patent Application Laid-Open No. 58-57012 (1983), the blast furnace gas is compressed to be a high-pressure gas, which is mixed with compressed air and burned, and the combustion gas is used to drive a gas turbine, thereby generatint electric power.
In such a power generation system, when an emergency shut down of the gas turbine is required, for example, when a power transmission system is failed and a shut-down interlocking device is operated, it is necessary to cut off the supply of the fuel to the gas turbine, for the shut-down of the gas turbine.
For the purpose of the emergency shut-down of the gas turbine, it is necessary for a gas supply shut-off valve to be located as close as possible to the inlet to the gas turbine. It is also necessary that the shut off operation can be performed at a sufficiently high speed.
The rapid closing action (for instance, in 0.5 to 1 sec) of the gas supply shut-off valve closes the passage on the discharge side of a gas compressor continuously discharging the high-pressure gas, resulting in a rapid increase in the discharge pressure. In practice, therefore, a bypass pressure reducing valve for releasing the high pressure gas into a bypass line must be rapidly opened simultaneously with the rapid closing action of the gas supply shut-off valve so that the passage on the discharge side of the gas compressor is not colsed.
The high-pressure gas on the discharge side of the gas compressor has, for instance, a pressure of about 12 kg/cm.sup.2 G and a temperature of about 350.degree. C. The pressure of the high pressure gas is lowered by the pressure reducing valve. Then, the gas is further lowered in pressure by a gas cooler disposed on the downstream side of the valve and is, simultaneously, cooled by a water spray, and the resultant low-pressure gas having a pressure of about 0.1 kg/cm.sup.2 G and a temperature of about 50.degree. C. is returned into the industrial by-product gas pipeline.
The pressure of the gas thus returned is not determined by only the characteristics of the gas compressor, bypass pressure reducing valve and gas cooler, but is further affected by the volume of the industrial by product gas pipeline and the tide of the industrial by-product gas at the moment of the emergency shut-down. Moreover, the pressure of the gas is affected also by the installation position of a gasholder and by the volume absorption rate of the gasholder.
For instance, where the quantity of gas consumed at the ga turbine is greater than the quantity of blast furnace gas generated from the blast furnace, it is difficult to control the pressure at the outlet of the gas cooler to or below a predetermined value at any time, as upon the emergency shut-down of the gas turbine.
Upon the rapid closing action of the gas supply shut-off valve for the emergency shut-down of the gas turbine mentioned above the gas turbine comes to be stopped. The gas compressor, on the other hand, continues rotating due to inertia for a while; therefore, until fully stopped, the compressor continues discharging the gas at a pressure matching the dishcarge resistance. Particularly, at the moment (a few seconds) of the emergency shut-down of the gas turbine, the gas compressor continues discharging the gas at the same rate as immediately before the emergency shut-down.
Therefore, when the bypass pressure reducing valve is rapidly opened in conjunction with the rapid closing of the gas supply shut-off valve for the gas turbine, both the high-pressure gas accumulated in the high pressure gas piping on the discharge side of the compressor and the gas discharged continuously from the compressor are returned into the industrial by-product gas pipeline. In the industrial by-product gas pipeline, therefore, not only the flow of gas toward the gas turbine is shut off but the gas will be caused to flow backward.
This phenomenon makes it more difficult to recognize the condition of pressure setting in the industrial by-product gas system including the gasholder.
For instance, the gas absorption rate limit of the gasholder may be exceeded and the gasholder be broken. Further, sealing water contained in drain discharge seal pots disposed at several positions of the industrial by-product gas pipeline may be blown out, leading to a gas leakage accident.
For effective use of gases, it is practiced to perform calorific value control by appropriately mixing different by-product gases at positions in an industrial by-product gas piping. For instance, blast furnace gas having a lower calorific value is mixed with coke oven gas having a higher calorific value to get a proper calorific value of the mixed gas, thereby matching the calorific value of the gas with the characteristics of the part at which the gas is used. Such a calorific value control through mixing of gases is performed by utilizing a low pressure difference of 500 to 1000 mm Hg, and is therefore heavily influenced by the above-mentioned disturbance in the gas pressure in the industrial by-product gas system.
In view of the above problems it may be contemplated for such a gas turbine plant system to reduce the quantity of the gas returned at a lowered pressure into the low-pressure gas piping upon the closure of the gas supply shut-off valve, by reducing the internal volume of the high-pressure gas piping from the compressor to the gas turbine through shortening the piping or reducing the diameter of the piping. Such an approach, however, involves resrtrictions on layout or increase the flow resistance in the high-pressure gas piping, and is therefore impracticable.